In general, threaded joints are used for the tubing and casing when an oil well is drilled. When these threaded joints are used in an actual environment, they are given a compound force including an internal and external pressure, an axial force and a bending force. Accordingly, these threaded joints are required not to leak or be damaged even when the compound force is given to them. On the other hand, when the tubing and casing are set in an oil well, threaded joints to join the oil well pipes are unfastened after they have been once fastened. In general, according to API (American Petroleum Institute), it is desired that no galling occurs in the threaded joints even when the tubing is fastened and unfastened 10 times and the casing is fastened and unfastened 3 times. In order to meet the above requirements, it is common at present that the threaded joints are fastened after compound grease stipulated in API BUL5A2 has been coated on the joints. In this case, compound grease works to ensure the galling resistance and enhancing the sealing property of the threaded joints.
In order to further enhance the sealing property of the threaded joints, special threaded joints having a contact portion in which metal comes into contact with metal, that is, premium joints have been actively developed. As disclosed in Japanese Examined Patent Publication Nos. 59-44552 and 5-41876, premium joints having sealing sections of various shapes have been invented. According to the above inventions, gas-sealing properties of the above joints are enhanced to the same level as that of the yield strength of the steel pipe. However, in order to further enhance the sealing property, a surface pressure higher than the yield strength of the mother metal must be given to the metal contact portion of the threaded joint. Accordingly, galling incapable of being repaired tends to occur in the threaded joint.
Therefore, the inventors actively investigated the prevention of galling.
There are provided various measures to prevent the occurrence of galling, which will be described as follows. Compound grease is made to appropriately contain powder of a heavy metal such as zinc, lead or copper; alternatively compound grease is made to appropriately contain powder of an inorganic compound such as mica; a shape of the sealing section is devised so that the local surface pressure on the threaded joint can be reduced as disclosed in Japanese Unexamined Patent Publication Nos. 62-209291 and 4-277392; a characteristic of the sealing surface is controlled as disclosed in Japanese Examined Utility Model Publication No. 6-713; and galling resistance is enhanced by conducting surface treatment on the threaded joint as disclosed in Japanese Examined Patent Publication No. 3-78517 and Japanese Unexamined Patent Publication Nos. 5-117870, 62-258283, 60-26695, 58-31097, 58-17285, 61-124792 and 61-136087. The aforementioned techniques are respectively effective to prevent the occurrence of galling. Especially when an appropriate surface treatment is conducted on the threaded joint and an appropriate compound grease is used to coat the threaded joint, galling resistance can be enhanced to a sufficiently high level to be put into practical use.
According to Japanese Examined Patent Publication No. 3-78517, threaded joints used for steel pipes to drill an oil well are coated with the coating of resin in which powder of molybdenum disulfide is dispersed and mixed. However, according to the technique disclosed in the above patent publication, the resin coating layer is formed in a range within the surface roughness of a portion in which metal comes into contact with metal. In the above technique, consideration is given to the coating of compound grease, that is, the target of the above technique is to enclose compound grease in the small spaces formed by irregularities on the surface of the threaded joint. Accordingly, when the threaded joint is fastened under the condition of no lubricant, metal is selectively contacted with metal. Therefore, it is impossible to stably provide a galling resistance. Further, in the above technique, consideration is not given to a surface preparation for preventing the deterioration with time, but consideration is given only to the surface roughness. Therefore, it is impossible to provide a stable galling resistance by the above technique over a long period of time.
Further, Japanese Unexamined Patent Publication No. 6-10154 discloses a technique in which a relation between the maximum surface roughness before surface treatment and the thickness of surface coating is stipulated. However, an object of the above technique is to enhance the sealing property by reducing a clearance formed between metal and metal in the metal contact portion. In the above patent publication, the effect of compound grease is described, however, no description is made about the galling resistance under the condition of no lubricant. As described above, by the metallic surface treatment described in the example, it is impossible to provide a galling resistance under the condition of no lubricant.
In the above circumstances, investigation has been recently made into grease to be coated on threaded joints. According to Japanese Unexamined Patent Publication Nos. 63-210487 and 6-11078, it is disclosed that the performance of grease is deteriorated when high pressure is given to grease coated on a threaded joint in the process of making up the threaded joint, that is, in the process of fastening the threaded joint. Further, environmental problems are discussed which are caused by heavy metal contained in compound grease. Therefore, it is attempted to develop compound grease containing no heavy metal and put it on the market. According to API RP5C5 which was enacted in 1991, it is stipulated to evaluate effects provided by a quantity of grease and a pressure given to grease. To make the matter worse, when compound grease is coated at the site, the working environment is deteriorated, and at the same time the working efficiency is lowered. In view of the above circumstances, it is required to provide an epoch-making threaded joint by which galling resistance can be ensured without using any compound grease. In spite of the above circumstances, it is inevitable to use compound grease when a threaded joint is joined. The reason is that the galling resistance is greatly deteriorated when the threaded joint is joined under the condition of no lubricant.
According to U.S. Pat. No. 4,692,988, it is described to use molybdenum disulfide as a solid lubricant to be coated on a threaded joint and then the threaded joint is bound by PTFE, and also a method of coating PTFE by carrier is described. It is recognized that the above technique can provide effects when it is used in a threaded joint together with liquid lubricant. When the above technique is used under the condition of no lubricant, which is the final object of the present invention, it impossible for the above technique to provide a sufficient effect. The reason is described below. Especially when a threaded joint is sealed, sliding motions are repeatedly conducted in the threaded joint while a surface pressure higher than the yield strength of the pipe body is given onto the threaded joint. Therefore, galling occurs in the thread portion of the joint.
In order to solve the above problems, the present inventors made investigation into the surface preparation and finally found a means for providing the galling resistance under the condition of no lubricant. The means is described as follows. After the sealing portion and the thread portion of a threaded joint have been subjected to an appropriate treatment of phosphate coating, a solid lubricant coating is provided on the phosphate coating.
U.S. Pat. No. 4,414,247 discloses an example in which solid lubricant is applied to a threaded joint in the same manner as described above. According to the above invention, after a threaded joint has been subjected to the surface preparation of blasting, molybdenum disulfide is coated and baked on the joint together with resin. However, when the surface preparation of blasting is conducted on the joint, it is impossible to provide a sufficiently high galling resistance under the condition of no lubricant. On the other hand, when the joint is subjected to the treatment of phosphate coating, no galling occurs even under the condition of no lubricant as described in the example, and it is possible to make up and break out the joint more than 10 times.
The reason why the galling resistance can be enhanced by the phosphate coating layer is described as follows. Irregularities and voids formed on the phosphate coating layer enhance the anchor effect of resin. Therefore, even after sliding has been repeatedly conducted, a thin lubricant coating remains on an upper surface of the phosphate coating layer.